The use of metal detergents in trunk piston engine oils (TPEO's) is well known and is described in the art. See, for example, U.S. Pat. No. B2-7,053,027. In “The Benefits of Salicylate Detergents in TPEO Applications with a Variety of Base Stocks”, Proceedings of the 7th International Symposium on Marine Engineering, Tokyo, Oct. 24 to 28, 2005, criteria for the selection of the best detergent for TPEO's are discussed. The paper states that there is a move towards the use of more Group II base oils in marine lubricants in various regions of the world, based on availability and cost.
The paper concludes that salicylate has advantage over other other detergents as an asphaltene dispersant in both Group I and Group II base stocks. However, in section 1.4, the paper states that overbased detergents show lower stability in Group II oils.
The present invention ameliorates the stability problem in Group II oils by employing specific ratios of overbased metal carboxylate detergents of defined basicity index.